"They pump out a radar beam and measure the beam coming back. The problem is the beam can get confused and pick up things fluttering in the distance like a bird or piece of litter."It all depends on the individual who"s pointing it and their interpretation of the results. I"ve pointed it at a Hoover before and it clocked a speed of 120 miles an hour."

Source: PEOPLE INVESTIGATION: 18mph TREE NICKED (The People (UK), 2/20/2005)